Allegheny was Pennsylvania's first to bear a Native American name, being named after the Allegheny River. The word "Allegheny" is of Lenape origin, with uncertain meaning. It is usually said to mean "fine river", but sometimes said to refer to an ancient mythical tribe called "Allegewi" that lived along the river before being destroyed by the Lenape.[3]

Little is known of the region's inhabitants prior to European contact. During the colonial era, various native groups claimed or settled in the area, resulting in a multi-ethnic mix that included Iroquois, Lenape, Shawnee, and Mingo. European fur traders such as Peter Chartier established trading posts in the region in the early eighteenth century.

Since most of the towns during that era were developed along waterways, both the French and the British desired control over the local rivers. Therefore, the British sent Major George Washington to expel the French from their posts, with no success. Failing in this objective, he nearly drowned in the ice-filled Allegheny River while returning. The English tried in 1754 to again enter the area. They sent 41 Virginians to build Fort Prince George. The French learned of the plan and sent an army to capture the fort, which they then resumed building with increased fortification, renaming it Fort Duquesne.

The loss cost the English dearly because Fort Duquesne became a focal point of the French and Indian War. The first attempt to retake the fort, the Braddock Expedition, failed miserably.[4] It was recaptured in 1758 by English forces under General John Forbes; he had it destroyed after its capture. They then built a new fort on the site, including a moat, and named it Fort Pitt. The site is now Pittsburgh's Point State Park.

Both Pennsylvania and Virginia claimed the region that is now Allegheny County. Pennsylvania administered most of the region as part of its Westmoreland County. Virginia considered everything south of the Ohio River and east of the Allegheny River to be part of its Yohogania County and governed it from Fort Dunmore. In addition, parts of the county were located in the proposed British colony of Vandalia and the proposed U.S. state of Westsylvania. The overlapping boundaries, multiple governments, and confused deed claims soon proved unworkable. In 1780 Pennsylvania and Virginia agreed to extend the Mason–Dixon line westward, and the region became part of Pennsylvania. From 1781 until 1788, much of what had been claimed as part of Yohogania County, Virginia, was administered as a part of the newly created Washington County, Pennsylvania.

Allegheny County was officially created on September 24, 1788, from parts of Washington and Westmoreland counties. It was formed due to pressure from settlers living in the area around Pittsburgh, which became the county seat in 1791. The county originally extended north to the shores of Lake Erie; it was reduced to its current borders by 1800.

Until 1 January 2000, Allegheny County's government was defined under Pennsylvania's Second Class County Code. The county government was charged with all local activities, including elections, prisons, airports, public health, and city planning. All public offices were headed by elected citizens. There were three elected county commissioners.

On 1 January 2000 the Home-Rule Charter went into effect. It replaced the three elected commissioners with an elected chief officer (the County Executive), a county council with 15 members (13 elected by district, two elected county-wide), and an appointed county manager. The changes were intended to maintain a separation of powers between the executive and legislative branches while providing greater citizen control.

County Medical Examiner office

The county has 130 self-governing municipalities, the most in the state (Luzerne is second with 76).[6] The county has one Second Class City (Pittsburgh) and three Third Class Cities (Clairton, Duquesne, and McKeesport).

A 2004 study found the county would be better served by consolidating the southeastern portion of the county (which includes many small communities with modest economies) into a large municipality ("Rivers City") with a combined population of approximately 250,000.[7]

As of November 7, 2017, there was 921,861 registered voters in the county; a majority were Democrats. There were 536,248 registered Democrats, 258,340 registered Republicans, 120,994 voters registered to other parties, 4,929 to the Libertarian Party and 1,350 voters registered to the Green Party.[11]

The Republican Party had been historically dominant in county-level politics in the 19th and early 20th centuries; prior to the Great Depression, Pittsburgh and Allegheny County had been majority Republican. Since the Great Depression on the state and national levels, the Democratic Party has been dominant in county-level politics and is the Democratic stronghold of western Pennsylvania. In 2000, Democrat Al Gore won 56% of the vote and Republican George W. Bush won 41%. In 2004, Democrat John Kerry received 57% of the vote and Republican Bush received 42%. In 2006, Democrats Governor Ed Rendell and Senator Bob Casey, Jr. won 59% and 65% of the vote in Allegheny County, respectively. In 2008, Democrat Barack Obama received 57% of the vote, John McCain received 41%, and each of the three state-row office winners (Rob McCord for Treasurer, Jack Wagner for Auditor General, and Tom Corbett for Attorney General) also carried Allegheny.

At the census[18] of 2000, there were 1,281,666 people, 537,150 households, and 332,495 families residing in the county. The population density was 1,755 people per square mile (678/km²). There were 583,646 housing units at an average density of 799 per square mile (309/km²). The racial makeup of the county was 84.33% White, 12.41% Black or African American, 0.12% Native American, 1.69% Asian, 0.03% Pacific Islander, 0.34% from other races, and 1.07% from two or more races. About 0.87% of the population were Hispanic or Latino of any race. 20.0% were of German, 15.0% Italian, 12.7% Irish, 7.5% Polish and 5.1% English ancestry according to Census 2000. 93.5% spoke English and 1.3% Spanish as their first language.

There were 537,150 households out of which 26.40% had children under the age of 18 living with them, 46.10% were married couples living together, 12.40% had a female householder with no husband present, and 38.10% were non-families. Some 32.70% of all households were made up of individuals and 13.20% had someone living alone who was 65 years of age or older. The average household size was 2.31 and the average family size was 2.96.

The age distribution of the population shows 21.90% under the age of 18, 8.50% from 18 to 24, 28.30% from 25 to 44, 23.40% from 45 to 64, and 17.80% who were 65 years of age or older. The median age was 40. For every 100 females, there were 90.00 males; for every 100 females age 18 and over, there were 86.20 males.

In the late 18th century farming played a critical role in the growth of the area. There was a surplus of grain due to transportation difficulties in linking with the eastern portion of the state. As a result, the farmers distilled the grain into whiskey, which significantly helped the farmers financially.

The area quickly became a key manufacturing area in the young nation. Coupled with deposits of iron and coal, and the easy access to waterways for barge traffic, the city quickly became one of the most important steel producing areas in the world. Based on 2007 data from the US Army Corps of Engineers, Pittsburgh is the second (after Laredo, Texas) busiest inland port in the nation.

US steel production declined late in the 20th century, and Allegheny County's economy began a shift to other industries. It is presently known for its hospitals, universities, and industrial centers. Despite the decline of heavy industry, Pittsburgh is home to a number of major companies and is ranked in the top ten among US cities hosting headquarters of Fortune 500 corporations, including U.S. Steel Corporation, PNC Financial Services Group, PPG Industries, and H. J. Heinz Company.

The Commonwealth of Pennsylvania has 36 Approved Private Schools including the Charter Schools for the Blind and Deaf. The private schools are licensed by the State Board of Private Academic Schools. They provide a free appropriate special education for students with severe disabilities. The cost of tuition for these schools is paid 60% by the state and 40% by the local school district where the student is a resident. Pennsylvania currently has four PA chartered and 30 non-charter APSs for which the Department approves funding. These schools provide a program of special education for over 4,000 day and residential students. Parents are not charged for the services at the school.[20] In 2009, the Pennsylvania Department of Education budgeted $98 million for tuition of children in approved private schools and $36.8 million for students attending the charter schools for the deaf and blind.[21] The majority of these schools are located in the southeastern region and southwestern region of Pennsylvania.

ACLD Tillotson School, Pittsburgh – Tuition rate $38,804

The Day School at The Children's Institute, Pittsburgh – Tuition rate $55,217

These are state-designated before- and after-school program providers. They receive state funding through grants. CCLCs provide academic, artistic and cultural enhancement activities to students and their families when school is not in session.[22]

Census-designated places are geographical areas designated by the US Census Bureau for the purposes of compiling demographic data. They are not actual jurisdictions under Pennsylvania law. Other unincorporated communities, such as villages, may be listed here as well.

Carrick Borough – now the neighborhood of Carrick. Formed out of Baldwin Township in 1904, this borough existed until it was annexed by Pittsburgh in 1927. It was named for Carrick-on-Suir, Ireland. Some of the area's manhole covers still bear the Carrick Borough name.

McClure Township – McClure was formed in 1858 from the section of Ross Township adjacent to Allegheny City. In 1867 McClure, along with sections of Reserve Township, was incorporated into Allegheny City. The McClure section of this annexation became Wards 9 (Woods Run Area) and 11 (present day Brighton Heights) in the City of Pittsburgh.

Mifflin Township- comprised the modern day communities of Whitaker, West Mifflin, West Homestead, West Elizabeth, Pleasant Hills, Munhall, Lincoln Place, Jefferson Hills, Homestead, Hays, Duquesne, Dravosburg, Clairton and part of Baldwin.[26]

Patton Township – was in east central part of the county, north of North Versailles Township, east of Wilkins and Penn Townships, and south of Plum Township. In U.S. census for 1860–1880. In 1951 it became incorporated as the borough of Monroeville.

Northern Liberties Borough – in what is now the Strip District of Pittsburgh. The borough was annexed to Pittsburgh in 1837 as the first addition to the city's original territory.

Peebles Township – included most of what is now the eastern part of the city of Pittsburgh from the Monongahela River in the south (today's Hazelwood) to the Allegheny River in the north. It was subdivided into Collins and Liberty townships, all of which were incorporated into Pittsburgh in 1868.

Pitt Township

St. Clair Township – stretched from the Monongahela River south to the Washington County line. It divided into Lower St. Clair, which eventually became part of the City of Pittsburgh, Dormont, Mount Lebanon, and Upper St. Clair.